One might expect that the quality of content received over a digital channel degrades smoothly as channel quality decreases. In actuality however, the quality of the received content suddenly and precipitously drops when channel quality falls below a certain critical point. Those skilled in the art of digital communications call this the “cliff effect.” Conversely (and counterintuitively), if the channel quality is above the critical point, the quality of the received content is no better than at the critical point.
While the cliff effect is inherent in digital communications, it has not presented a practical problem until recently, because most digital content was heretofore delivered point-to-point (unicast) over networks having fairly constant channel quality. Transmitters of point-to-point communications have the benefit of sensing the characteristics of the channel to be employed. Based on those characteristics, the transmitters can use conventional source coding and channel coding techniques to encode the content for those characteristics and mitigate the cliff effect.
However, today's digital networks (e.g., dedicated content distribution networks and mobile and wireless networks) are tasked with delivering content over diverse channel conditions to heterogeneous users. As a result, the cliff effect has become a significant impediment to efficient multicast communications. Consequently, when streaming content is simultaneously delivered to multiple users, not only do the recipients whose channel quality is below the critical point receive unusable streams, the recipients whose channel quality is above (even significantly above) the critical point receive no benefit from the enhanced quality of their channels.